Carcinogenic doses of 7,12-dimethylbenz(Alpha)anthracene (DMBA) failed to induce mammary carcinomass in rats receiving dibutyryl cyclic AMP (DBcAMP). The inhibitory effect of DBcAMP on the mammary carcinogenesis correlated with the antagonistic action of DBcAMP on the DNA-binding of DMBA in vivo. The antagonistic interrelation between DBcAMP and DMBA in their DNA-binding was also demonstrable with mammary epethelial cells in culture. The results suggest that DBcAMP may prevent the induction of mammary carcinogenesis by blocking the binding to DNA of the carcinogen.